Fiberous shield means for a magnetic focus coil

ABSTRACT

A magnetic focus system in which a cylindrical wire coil is enclosed by a magnetic shield in the shape of a hollow cylinder. An annular gap is provided in the interior wall of the shield. The shield is formed of a plurality of substantially parallel, magnetically conducting fibers in a matrix of dielectric material oriented with respect to the magnetic field so that the permeability of the shield is anisotropic.

United States Patent 91 Sawyer 11] 3,745,494 1451 July 10,1973

1 1 FIBEROUS SHIELD MEANS FOR A MAGNETIC FOCUS COIL [75] Inventor: Carleton E. Sawyer, Littleton, Mass.

[73] Assignee: Display Components, Inc., Littleton,

Mass.

[22] Filed: Mar. 7, 1972 [21] Appl. No.: 232,588

[52] [1.8. CI. 335/210, 250/495 [51] Int. Cl. I-I0lf 7/00 [58] Field of Search 335/210, 297;

[56] References Cited UNITED STATES PATENTS 3,008,044 11/1961 Buchhold ..335/21OX 3/1970 Katagiri et al. 335/210 2/1972 Kasai et a]. 250/495 D Primary Examiner-George Harris Attorney-Robert .l. Schiller et al.

[57] ABSTRACT A magnetic focus system in which a cylindrical wire coil is enclosed by a magnetic shield in the shape of a hollow cylinder. An annular gap is provided in the interior wall of the shield. The shield is formed of a plurality of substantially parallel, magnetically conducting fibers in a matrix of dielectric material oriented with respect to the magnetic field so that the permeability of the shield is anisotropic.

8 Claims, 3 Drawing Figures FIBEROUS SHIELD MEANS FOR A MAGNETIC FOCUS COIL This invention relates to magnetic focus fields and more particularly to a precision magnetic lens system. The various magnetic lens systems known in the art typically comprise a cylindrical wire coil enclosed by a magnetic shield of relatively isotropic, magnetically permeable material having an annular air gap provided in the interior wall of the shield. A DC current passing through the coil produces a magnetic field which is channeled by the gap to provide a focus field. An important requirement of such systems is that the magnetic field strength be symmetrical around the gap; otherwise, the field will be non-uniform and will result in astigmatised focused beams. Hitherto, attempts to provide symmetrical fields have stressed selecting and treating the core material in an attempt to obtain material having a substantially constant permeability at every point. Such an approach has several drawbacks, the major problem being that it is extremely difficult, if not impossible, to control the permeability so that is is sufficiently constant.

A primary object of the present invention is to provide a magnetic focus system which overcomes the foregoing problems and further provides a well controlled, symmetrical field. Other objects of the present invention are to provide a magnetic focus system comprising a solenoidal coil or core having a cylindrical shield surrounding the coil, the shield having a permeability, in the direction of the flux of the field of the coil, substantially greater than the permeability transverse to the flux direction.

To effect the foregoing and other objects generally, the present system comprises a cylindrical wire coil enclosed by a cylindrical magnetic shield having an annular gap in the inner cylindrical wall thereof. The shield is formed of a plurality of substantially parallel, magnetizable fibers oriented with respect to the core so as to provide an anisotropic magnetic path wherein the permeability generally along the field direction with respect to the coil is substantially greater than the transverse permeability.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the method and apparatus involving the several steps and the relation and order of one or more of such steps with respect to each of the others and the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a cutaway perspective view of a magnetic focus system embodying the invention; and

FIG. 2 is a cross-sectional view of a portion of one embodiment of FIG. I; and

FIG. 3 is another cross-sectional view of a portion of an alternative embodiment of FIG. 1.

Referring now to FIG. 1 there is shown an embodiment of the present invention which includes a cylindrical element 20. Element 20 is formed as a coil of electrically conducting material such as copper or aluminum wire in which all conductors are substantially parallel to one another and formed into a plurality of circular loops. Cylindrical element 20 is adapted to be connected to a DC power supply (not shown), and is enclosed by a magnetic shield 22 shaped in the form of a hollow cylinder. Shield 22 is mounted with its central axis collinear with the central axis of the coil. On the interior wall of shield 22 and extending radially inwardly is a gap 24. Gap 24 may be an air gap in shield 22 as shown in FIG. 2, or may comprise a ring 25, as shown in FIG. 3 of substantially non-magnetic (i.e., low permeability) material mounted with its central axis collinear with the central axis of shield 22. Gap 24 serves to provide a magnetic focusing field produced by flow of electrical current through element 20.

As shown in detail in FIG. 2, magnetic shield 22 comprises a coating or sheath surrounding or covering element 20 completely except with respect to annular gap 24 which is preferably positioned around the interior cylindrical periphery of element 20 adjacent one end of the cylindrical form of the latter. Shield 22 comprises a plurality of fibers 26 which are formed of a material having a high magnetic permeability and preferably a relatively low magnetic remanence, such as soft iron, a number of iron alloys, or the like. The term fiber" as used herein, is intended to mean an elongated element, preferably of substantially constant cross-sectional configuration and dimension, and having a length-towidth ratio much greater than unity and preferably an order of magnitude or more. In a typical configuration, fibers 26 are formed of 0.030 inch soft iron wire.

Fibers 26 are preferably embedded in a matrix of a high dielectric material such as glass, epoxy resin, or other synthetic plastic which serves to provide the mechanical support for maintaining the fibers in a fixed orientation, provides a relatively low magnetic permeability path from fiber to fiber, and electrically insulates the fibers from one another and from the electrically conductive coil from member 20. Within this matrix, fibers 26 are oriented to be substantially parallel with one another and with the cylindrical axis of element 20 in a first array 30 distributed around the inner cylindrical periphery of element 20 and in a second similar array 32 distributed around the outer cylindrical periphery of element 20. It will thus be seen fibers 26 of arrays 30 and 32 lie with their long axes substantially parallel with the lines of force of a magnetic field associated with core 20 when the latter is energized by passage of an electrical current therethrough. To complete a magnetic circuit between the inner and outer arrays of fibers 26 and also to complete the magnetic shielding around element 20, there are provided end caps 34 and 36 shown in FIG. 2 as simply a pair of rings formed of high permeability, low remanence material in physical contact with ends of one or more the arrays of fibers.

Alternatively as shown in FIG. 3, rings 34a and 36a are formed of fibers such as 26 which are oriented radially with respect to the cylindrical axis of element 20. The embodiment of FIG. 3 can be formed either by first orienting a plurality of fibers in a radial direction, embedding them in a matrix and cutting a ring out of the composite. The outer and if necessary the inner edges of the ring are then bevelled to expose fiber ends and the ring is pressed against and sealed to the inner and outer arrays of fibers which have also had edges bevelled to provide the desired magnetic contact. Alternatively, one can form the embodiment of FIG. 3 simply by winding a single fiber 26 as a continuous strand in I.

a torus around the core material, filling the torus interstices with a synthetic plastic and then cutting away the internal surface of the torus to form gap 24.

Regardless of the manner of contruction of the device of the invention, all of the embodiments operate in substantially the same manner. A current is passed through the conductors of element 20 which are, of course, electrically insulated from one another so as to provide a multi-turn coil. The magnetic potential provided by the current creates a magnetic field which would normally exhibit its highest field intensity within the interior of the coil and have a substantial field configuration extending well outside of the external periphery of the coil. By virtue of the magnetic shielding, the external field is substantially attenuated. It will be observed that because of the orientation of fiber 26 in arrays 30 and 32, there is provided a relatively high permeability path in the direction of the adjacent magnetic field which tends to trap or channel the field in that direction. However, because the dimension of those fibers in the transverse direction with respect to the field of the coil is very small and the magnetic material is essentially subject to a large number of discontinuities, the bulk magnetic permeability property of arrays 30 and 32 in the transverse direction will be lower than in the direction of the field flux.

The magnetic field configuration provided by the low permeability gap 24 has, as well known in the art, by virtue of the enclosing magnetic shield 22, a relative minimization of the effects of field irregularities. In the present invention, however, even if the magnetic permeability of the fibers varies somewhat from fiber to fiber, if the variation is random, it will tend to be statistically damped, and the total effect will be that of a magnetic shield having a substantially homogeneous permeability in the direction of the field flux and a substantially lesser but equally homogeneous permeability in the transverse direction The nature of the shielding achieved by the present invention can be seen from the following analysis: As-

suming a well organized packing function where a two dimensional analysis can be considered as representative of the bulk characteristics of the shielding, one can postulate a model having a height of a fiber diameter and a width equal to ns where sis a uniform fiber center to fiber center spacing and n is the number of fibers. One can also postulate that all of fibers 26 have the same diameter D, are uniformly spaced a distance S apart (fiber axis-to-fiber axis), in a matrix of dielectric material, the fibers being semi-infinite in length. Now for fibers formed of material of permeability u, the average longitudinal permeability 11., in the direction of the fibers is:

(L) The bulk or average permeability transverse to the direction of the fibers will be approximated by:

#1 UD/M where (S D) is greater than zero, (S D),u will normally be so much larger than S, that the latter can be ignored and equation (5) can be approximated as:

t, S/S I) One can determine the ratio of permeabilities (Ra) then as To find the point of maximum permeability ratio one need only differentiate with respect to 5, set the derivative equal to zero and solve for S thus:

Substituting equation (6) into equation (4), the maximum permeability ratio is thus:

mur "M 16 Assuming a shielding comprising fibers of grainoriented soft iron having a permeability of approxiamtely 30,000, a diameter D of approximately 0.020 inch, and a fiber-to fiber spacing S of approximately 0.040 inch, a permeability ratio of nearly 6,000-to-l is obtained. Normally, the permeability 11. within a material may vary up to 20 percent. A table showing the effect of 10 percent and 20 percent variations in p. from a value of 30,000 is given below:

I i/I 5 From this table it is apparent that even though actual permeability may vary from fiber-to-fiber by 20 percent, the axial permeability will still be thousands of times greater than the transverse permeability. This results in a well controlled field substantially the same around the 360 of the gap.

Since certain changes may be made in the above apparatus and process without departing from the scope of the invention herein involved it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense.

What is claimed is:

1. A magnetic focus coil comprising in combination:

a substantially cylindrical wire coil;

a cylindrically shaped magnetic shield enclosing said coil; and having an annular gap in the interior cylindrical periphery thereof, said magnetic shield comprising a plurality of fibers of material of high permeability and low remanence oriented with respect to said coil so as to provide an anisotropic magnetic path wherein the bulk permeability of said shield along the direction of the lines of the magnetic field provided by excitation of said coil is substantially greater than the bulk permeability of said shield in a direction transverse to said lines. 2. A coil as described in claim 1 wherein said fibers are formed of an iron-bearing material.

3. A coil as described in claim 1 wherein said gap is an air gap.

4. A coil as described in claim 1 wherein said gap comprises a ring of non-magnetic material.

5. A coil as described in claim 1 wherein said fibers are disposed in a matrix of dielectric material.

6. A coil as described in claim 1 wherein said fibers are formed substantially as a single strand wound as an approximate toroid around said coil.

7. A coil as described in claim 1 wherein a first array of said fibers forms an outer portion of said shield disposed about the external cylindrical periphery of said coil,

a second array of said fibers forms an inner portion of said shield disposed about the inner cylindrical netic field. 

1. A magnetic focus coil comprising in combination: a substantially cylindrical wire coil; a cylindrically shaped magnetic shield enclosing said coil; and having an annular gap in the interior cylindrical periphery thereof, said magnetic shield comprising a plurality of fibers of material of high permeability and low remanence oriented with respect to said coil so as to provide an anisotropic magnetic path wherein the bulk permeability of said shield along the direction of the lines of the magnetic field provided by excitation of said coil is substantially greater than the bulk permeability of said shield in a direction transverse to said lines.
 2. A coil as described in claim 1 wherein said fibers are formed of an iron-bearing material.
 3. A coil as described in claim 1 wherein said gap is an air gap.
 4. A coil as described in claim 1 wherein said gap comprises a ring of non-magnetic material.
 5. A coil as described in claim 1 wherein said fibers are disposed in a matrix of dielectric material.
 6. A coil as described in claim 1 wherein said fibers are formed substantially as a single strand wound as an approximate toroid around said coil.
 7. A coil as described in claim 1 wherein a first array of said fibers forms an outer portion of said shield disposed about the external cylindrical periphery of said coil, a second array of said fibers forms an inner portion of said shield disposed about the inner cylindrical periphery of said coil and including low remanence, high permeability means for joining one corresponding pair of edges of said portions to complete a magnetic circuit between said pair of edges, and low remanence, high permeability means joined to the remaining edge of said first array and extending substantially to said gap.
 8. A coil as described in claim 7 wherein at least one of said low remanence, high permeability means comprises a plurality of said fibers positioned so that the long axes thereof are substantially parallel to said magnetic field. 